1. Field of the Invention
The invention relates to a method of measuring a periodically varying fluid flow in an object by means of magnetic resonance, which object is arranged in a steady magnetic field, said method including the following steps:
generating an excitation RF pulse in order to direct a reference magnetization in a plane extending transversely of the direction of the steady magnetic field,
applying a first additional gradient in a flow direction of the fluid flow in order to produce a phase shift in the reference magnetization,
generating a first additional RF pulse in order to direct the reference magnetization in the direction of the steady magnetic field,
generating a second additional RF pulse in order to direct a part of the reference magnetization in a transverse direction relative to the direction of the steady magnetic
applying a second additional gradient in the flow direction in order to cancel the phase shift caused by the first additional gradient in static material near the fluid flow, measuring a first MR signal, and
determining a displacement of the fluid from the measured first MR signal and a reference MR signal measured in advance. The invention also relates to a device for carrying out such a method.
2. Description of the Related Art
A method of the described kind is known from the article xe2x80x9cDiscrimination of 2 Different Types of Motion by Modified Stimulated-echo NMRxe2x80x9d, published by J. E .M. Snaar et al. in Journal of Magnetic Resonance 87, pp. 132-140, 1990. The known method can be used, for example for in vivo measurement of the fluid flow of cerebral spinal fluid (CSF) in the brain of a human or animal to be examined. The production of CSF in the brain can be determined on the basis of the displacement of the CSF in for example the aqueduct of Sylvius in the brain. Deviations between the actual production of CSF and predetermined standard values may constitute an aid in diagnosing given neural degenerative diseases, for example Alzheimer""s disease. According to the known method the excitation RF pulse is generated in conjunction with the application of a gradient which is directed in a first direction in order to excite spins in the volume of a fluid flow. In order to realize a motion-induced phase shift in the first MR signal to be measured, after the generating of the excitation RF pulse a gradient pair is applied and subsequently the first additional RF pulse is generated. The net displacement of the fluid flow can then be determined from the phase of the measured first MR signal and the phase of the reference MR signal measured in advance.
It is a drawback of the known method that a measurement of the net displacement of the fluid flow is inaccurate because of a large periodic component in the fluid flow which is due to the periodically varying blood pressure.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
It is an object of the invention to provide a method offering improved measurement of the net displacement of the periodically varying fluid flow. To achieve this, the method according to the invention is characterized in that near a phase of a first period of the periodically varying fluid flow the excitation RF pulse is generated, the first additional gradient is applied and the first additional RF pulse is generated, and that near a corresponding phase of a second period of the periodically varying fluid flow the second additional RF pulse is generated, the second additional gradient is applied and the first MR signal is measured. The principle of the invention is as follows: a reference is created by means of the generated magnetization of a measuring volume of the periodically varying fluid flow near the phase of the first period of the periodically varying fluid flow in a first position in an examination space; this magnetization is directed along the first axis and near the corresponding phase of the second period, in the same position in the examination space, a part of this magnetization of the measuring volume is used to generate the first MR signal. The measured first MR signal then contains information concerning the net displacement of the fluid flow in the interval between the corresponding phases of the first and the second period. The first MR signal and the generated reference then contain information as regards the net displacement of the measuring volume in the fluid flow. An example of such a reference is the marking of the magnetization directed in the transverse plane by realization of a phase shift.
A special version of the method according to the invention is characterized in that the first phase of the period of the periodic fluid flow corresponds to a zero-crossing of a flow velocity of the periodic fluid flow. The error in the phase shift which is due to the flow velocity is minimized by application of the first and the second additional gradients near the zero crossings of the first and the second periods.
A further version of the method according to the invention comprises determining the reference MR signal by performing, near a corresponding phase of a further third period of the periodically varying fluid flow, generating the excitation RF pulse, applying the first additional gradient, and generating the first additional RF pulse, and near a corresponding phase of a further fourth period of the periodically varying fluid flow, generating the second additional RF pulse, applying the second additional gradient, and measuring the reference MR signal, wherein a time integral of the first additional gradient applied in the third period is different from a corresponding time integral of the first additional gradient applied in the first period. The reference MR signal can be simply determined by repetition of the same pulse sequence.
A further version of the method according to the invention comprises determining the reference MR signal by generating a third additional RF pulse near a corresponding phase of a next period of the periodic fluid flow, applying a third additional gradient which has the same properties as the second additional gradient, and measuring the reference MR signal. The measurement of the first MR signal and the reference MR signal is thus integrated in one pulse sequence. A net displacement of the periodically varying fluid flow can thus be determined from only three successive zero crossings.
Another version of the method according to the invention in which the fluid flow varies with a period which corresponds to a period of a cardiac cycle of a human or animal to be examined, comprises measuring an ECG of the human or animal to be examined, and determining a reference from the measured ECG which corresponds to a phase in the period of the periodically varying fluid flow. The phases of the periods of the periodic fluid flow can be derived from the electrocardiogram (ECG), for example by determining the reference from the occurrence of an R-wave in the ECG and the phase in the period of the periodic fluid flow.
Another version of the method according to the invention comprising determining a zero crossing of the flow velocity of the periodically varying fluid flow by measurement of a flow velocity of the periodically varying fluid flow, wherein the reference also corresponds to the zero crossing of the flow velocity of the periodically varying fluid flow A known phase contrast MR angiography method is an example of a flow velocity measurement by means of magnetic resonance. This method is known, for example from the handbook xe2x80x9cMagnetic Resonance Imagingxe2x80x9d, published by M. T. Vlaardingerbroek, Springer-Verlag, pp. 294-295.
In another embodiment of the method according to the invention a gradient pair for measuring a fluid flow can thus be combined with a gradient for measuring the MR signal. Therefore, the invention also relates to a method of forming a two-dimensional velocity profile of a periodically varying fluid flow in an object which is arranged in a steady magnetic field, which method includes the following steps:
a) generating an excitation RF pulse in order to direct a reference magnetization in a transverse plane relative to the direction of the steady magnetic field,
b) applying a first additional gradient in a flow direction of the fluid flow in order to induce a phase shift in the reference magnetization,
c) generating a first additional RF pulse in order to direct the reference magnetization in the direction of the steady magnetic field,
d) generating a second additional RF pulse in order to direct a part of the reference magnetization in a transverse direction relative to the direction of the steady magnetic field,
e) applying a second additional gradient in the flow direction in order to cancel the phase shift caused by the first additional gradient in static material near the fluid flow,
f) measuring first MR signals, and
g) determining a phase image from the measured first MR signals and determining the two-dimensional velocity profile of the fluid from the phase image and a reference phase image.
This latter method according to the invention is characterized in that near a phase of a first period of the periodically varying fluid flow the excitation RF pulse is generated, the first additional gradient is applied and the first additional RF pulse is generated, and that near a corresponding phase of a second period of the periodically varying fluid flow the second additional RF pulse is generated, the second additional gradient is applied and the first MR signal is measured.
The invention also relates to a device for carrying out such a method. A device of this kind comprises means for sustaining a steady magnetic field, means for applying gradients, means for generating RF pulses to be applied to the object to be examined in the steady magnetic field, a control unit for controlling the application of gradients and the generating of RF pulses, and means for receiving and sampling magnetic resonance signals which are generated by pulse sequences containing the RF pulses and the gradients, wherein said control unit is arranged to generate an excitation RF pulse in order to direct a reference magnetization into a plane which extends transversely of the direction of the steady magnetic field, apply a first additional magnetic field gradient in a flow direction of the fluid flow in order to produce a phase shift in the reference magnetization, generate a first additional RF pulse in order to direct the reference magnetization into the direction of the steady magnetic field, generate a second additional RF pulse in order to direct a part of the reference magnetization into a transverse direction relative to the direction of the steady magnetic field, apply a second additional magnetic field gradient in the flow direction in order to cancel the phase shift caused by the first additional gradient, measure a first MR signal, and determine a displacement of the fluid from the measured first MR signal and a reference signal, wherein the control unit is further arranged so that, near a phase of a first period of the periodically varying fluid flow, the excitation RF pulse is generated, the first additional gradient is applied and the first additional RF pulse is generated, and, near a corresponding phase of a second period of the periodically varying fluid flow, the second additional RF pulse is generated, the second additional gradient is applied and the first MR signal is measured